A storage server is a type of network storage controller which is a special purpose data processing system used to store and retrieve data on behalf of one or more clients on a network. A storage server operates on behalf of one or more clients to store and manage data in a set of mass storage devices, such as magnetic or optical storage-based disks or tapes. In conventional network storage systems, the mass storage devices may be organized into one or more groups of drives (e.g., redundant array of inexpensive disks (RAID)). A storage server also typically includes internal memory that is used as a buffer cache, to speed up the reading and writing of data from and to (respectively) the main mass storage system. In conventional storage servers, this buffer cache typically is implemented the form of dynamic random access memory (DRAM).
It is desirable to improve the performance of any data processing system. In the case of a storage server, one way to accomplish this is by reducing the latency and increasing the random access throughput associated with accessing the storage server's main mass storage subsystem. In this regard, flash memory, particularly NAND flash memory, has certain very desirable properties. Flash memory generally has a very fast read access speed compared to that of conventional disk drives. Also, flash memory is substantially cheaper than conventional DRAM and is not volatile like DRAM.
However, flash memory also has certain characteristics that make it unfeasible simply to replace the DRAM or disk drives of a storage server with flash memory. In particular, a conventional flash memory is typically accessed by a host through a serial command interface. Because such an interface allows the flash memory only to receive one command (e.g., a read or write) at a time from the host, it can become a bottleneck in applications where low latency and/or high throughput is needed.
In addition, while flash memory generally has superior read performance compared to conventional disk drives, its write performance is generally not as good, especially for random writes. One reason is that each time a unit of flash memory is written, it must first be erased, which adds latency to write operations. Furthermore, flash memory tends to wear out after a finite number of erase operations.